This invention relates to a method and apparatus for optical fiber communications, and more particularly to digital communications over optical fibers at high data rates in the high gigabit range, e.g., five to eight gigabits per second.
Many ambitious wideband optical fiber communication projects with interactive services are presently under development throughout the world. The wide bandwidth is especially needed for high-resolution video transmission and distribution. In addition, microprocessor-based instrumentation and equipment numbering in the thousands, and spaced miles apart, may have a need to communicate with each other. This electronic trend will lead to automated factories and offices. Other high-data-rate systems will be associated with earth resource satellites and their new sophisticated high-data-rate sensors, e.g., synthetic aperture radar and imagers. In order to support this information explosion, gigabit per second optical fiber networks have to be constructed.
Optical fiber communication experiments in the 1 to 2 Gbit/s range have been performed in several laboratories using low-loss single-mode fibers. The major limitations have been in the modulation characteristics of the semiconductor lasers, which have been limited by turn-on delay, relaxation oscillations, and interpulse interference. The problem has been to utilize the large optical bandwidth of semiconductor lasers. Picosecond pulse generation, combined with optical multiplexing, is one method that may be utilized. Mode-locking techniques using a c.w. GaAlAs laser diode have reportedly produced very narrow pulses. Picosecond optical pulse generation from an r.f. modulated AlGaAs diode laser has also been reported. In this invention damped relaxation oscillation, single-mode semiconductor lasers biased 10-20% above threshold current have been used to increase the modulation bandwidth.